It is known that timepieces have hands. These hands consist of a bar with a length that is much larger than the width, which is itself much larger than the thickness. These hands comprise an opening for them to be pressed onto a staff in order to be mounted to pivot. In order to have hands that are fine and strong, it is provided to form them from a crystalline metal such as steel, brass, gold or even silicon or ceramic. These hands can be machined or cut out of a sheet by laser or water jet. They can also be moulded, sintered or formed by growing or depositing material. These hands are then used, for example, to indicate the hours, minutes and seconds, but are also used to perform certain functions such as chronograph functions or calendar functions.
These hands are in fact subjected to numerous stresses. One of these stresses is the weight of the hand itself. In fact, the hand is generally pressed onto its staff at one of its ends. Considering the small dimensions of a hand, it is completely normal for it to bend, if only slightly, under its own weight as a result of this. This weight stress is also applied to the unbalance that serves as counterweight for the hand.
The hand is also subjected to acceleration stresses. These stresses can be due firstly to the displacement controlled by the timepiece movement. This displacement is linked to the time display or to a function of said timepiece such as the chronograph function and can be retrograde. A return to zero of the hands occurs in the case of a retrograde display or during use of the chronograph function. This return to zero consists of an abrupt return of the hand to its initial position. During this return to zero operation the acceleration of the hand can reach 1.106 rad·s−2. Such an acceleration involves a high stress applied to the hand during acceleration and also during deceleration and stoppage of the hand.
Secondly, the stresses linked to acceleration can be due to a shock applied to the watch. In fact, when the watch falls, for example, it is subjected to acceleration. The energy accumulated during this fall is transferred to the hands upon contact of said watch with the ground. These shocks can then deform the hand or the unbalance, which can then cause problems during displacement of the hand.
A disadvantage of hands made from crystalline metal is their low mechanical resistance when high stresses are applied. In fact, each material is characterised by its Young's modulus E also referred to as modulus of elasticity (generally expressed in GPa), which characterises its resistance to deformation. Each material is also characterised by its elastic limit σe (generally expressed in GPa) that represents the stress beyond which the material is plastically deformed. It is thus possible, with given dimensions, to compare the materials by establishing for each the ratio of their elastic limit to their Young's modulus σe/E said ratio being representative of the elastic deformation of each material. Thus, the higher this ratio is, the higher the elastic deformation of the material. Typically, for an alloy such as Cu—Be the Young's modulus E is equal to 130 GPa and the elastic limit σe is equal to 1 GPa, which gives a σe/E ratio in the order of 0.007, i.e. a low ratio. Hands made of crystalline metal or alloy consequently have a limited elastic deformation. Consequently, during a return to zero or a shock the stresses applied to said hands can be so high that the hands deform plastically, i.e. they twist. This deformation thus poses a problem of readability and reliability of the information.
This deformation phenomenon is even more accentuated in the case of crystalline precious metals. In fact, these have even poorer mechanical characteristics. Precious metals have in particular a low elastic limit in the order of 0.5 GPa for alloys of Au, Pt, Pd and Ag compared to about 1 GPa for crystalline alloys classically used in the production of hands. Given that the elasticity modulus of these precious metals is in the order of 120 GPa, there results a σe/E ratio of about 0.004, that is to say an even lower figure than for non-precious alloys. The risks of deformation as a result of stresses applied during a significant acceleration such as a return to zero are thus increased. Consequently, a person skilled in the art is not encouraged to use these precious metals for the production of a timepiece hand. However, these precious metals are in high demand since they have a significant extra aesthetic value and exude a sense of superior quality.
In addition, current methods such as stamping, laser cutting or growth by deposition are limited. They do not allow three-dimensional hands to be formed. In fact, in the case of stamping or laser cutting the hands are formed from a sheet. The disadvantage in the case of the production of hands by LIGA type material growth is that the walls of the hands are straight and that no angled type of inclination is therefore possible.